Structural Characterization and Hypoglycemic Effects of Arabinogalactan-Protein from the Tuberous Cortex of the White-Skinned Sweet Potato (Ipomoea batatas L.)

An arabinogalactan-protein (WSSP-AGP) was isolated from the tuberous cortex of the white-skinned sweet potato (WSSP; Ipomoea batatas L.). It consists of 95% (w/w) carbohydrate and 5% (w/w) protein with high contents of hydroxyproline, alanine, and serine. Its sugar composition is α-l-Rha:α-l-Ara:β-d-Gal:β-d-GlcA in a molar ratio of 1.0:4.1:7.6:1.3. Its weight-average molecular weight was estimated to be 126800 g/mol by high-performance size exclusion chromatography coupled with multiangle laser light scattering. Structural analysis indicated that WSSP-AGP is a (1→3)-β-d-galactan highly branched at O-6 with (1→6)-β-d-galactan, in which the branched chains are substituted at the O-3 position with α-l-Araf-(1→ and α-l-Araf-(1→5)-α-l-Araf-(1→ and at the O-6 position typically with α-l-Rhap-(1→4)-β-d-GlcAp-(1→ as terminating groups. Continuous administration of WSSP-AGP to KKAy mice significantly lowered fasting plasma glucose levels. This indicates that WSSP-AGP plays an important role in the hypoglycemic effects of WSSP.